Field of the Invention
The present invention relates to a plant virus infection inhibitor, and a method for inhibiting plant virus infection using the same. The present invention is useful in the fields of agriculture, horticulture, and so forth.
Brief Description of the Related Art
Pathogenic germs that cause plant diseases are roughly divided into three kinds, i.e., filamentous fungi (molds), bacteria, and viruses.
The most prevalent causes of all plant diseases are those caused by filamentous fungi. Filamentous fungi are eucaryotes, and cause 73% of all the plant diseases. However, filamentous fungi have cell structures, life cycles, etc. significantly different from those of plants and animals, and it is easy to develop agricultural chemicals which are selectively and highly effective against specific filamentous fungi. In contrast, although viral diseases cause only about 5 to 7% of all plant diseases, these pathogens are made up of nucleic acids and proteins which are basic substances for life. Viruses proliferate by utilizing the replication, transcription and translation mechanisms of host plants, and therefore it is extremely difficult to develop a substance that selectively or specifically inactivates a certain virus. Accordingly, it is generally considered to be difficult to control viral diseases, and once a viral disease develops, it cannot be cured. Furthermore, if any measures are not taken, it can serve as a new source of infection, and cause further spread of the disease. Although there have been a lot of attempts aiming at control of viral diseases, such as blocking the route of infection using a pesticide or the like, removal of a plant suffering from a disease at an early stage of the disease, and breeding of a plant utilizing a resistance gene, RNA interference, or the like (creation of genetic recombinant), control of viral diseases is currently still very difficult in fact.
For control of plant diseases, the classification or identification of pathogenic viruses at the genus or species level is extremely important. This is because, for viruses of the same genus or species, analogy of vector or characteristics of the viruses may be contemplated, and thereby it becomes possible to choose a method for controlling viral disease or measures against the disease to a certain extent.
An example of a method of controlling viral diseases that has been put into practical use is a method of utilizing the interferential action of viruses. The interferential action refers to a phenomenon that a plant infected with a certain virus is protected from the secondary infection of the virus. Development of attenuated viruses also advances (Japanese Patent Laid-open (Kokai) No. 1-281079), and seedlings inoculated with attenuated CMV beforehand are partly commercialized. However, the serious drawback of this method is that this method is effective only for closely related viruses having highly homologous nucleic acid sequences.
A plurality of substances showing antiviral effect have been reported to date. For example, many substances such as the polysaccharides produced by the Candida famata T1 strain or a mutant thereof (Japanese Patent Laid-open No. 2001-72521), proteins derived from Phytolacca americana (Japanese Patent Laid-open No. 5-137580), acidic high molecular polysaccharides produced by basidiomycetes belonging to the genus Fomes (Japanese Patent Publication (Kokoku) No. 6-21122), high molecular polysaccharides produced in Schizophyllum fungus culture (Japanese Patent Laid-open No. 1-272509), alkyl diethylenetriaminoacetates and salts thereof (Japanese Patent Laid-open No. 2-121903), Camellia sinensis saponin (Japanese Patent Laid-open No. 7-25718), and polylysine (Japanese Patent Laid-open No. 63-2901) have been reported, however, they have not widely spread at present.
At present, only one kind of antiviral agent, lentemin (trade name of Noda Shokukin Kogyo Co., Ltd.), is registered as an agricultural chemical in Japan (agricultural chemical registration numbers, 15584, 17774, 19439, and 19440), except for attenuated viruses and soil disinfectants. The main ingredient of lentemin is a culture medium extraction component of a shiitake mushroom mycelium produced by culturing mycelia of shiitake mushroom on a solid medium, crushing the medium in which the mycelia spreads in the whole medium, and subjecting the whole medium to extraction. Probably for this reason, the price thereof per unit volume is comparatively expensive. Moreover, the recommended dilution rate of the lentemin solution for controlling viral mosaic disease is as low as 1 to 10 times (Yoneyama et al., Noyaku Binran (Agricultural Chemical Handbook), 10th edition, pp. 387-389, Rural Culture Association Japan), and therefore the cost required for spraying it per unit cultivation area is also extremely higher as compared to other common agricultural chemicals.
A virus infection inhibitor containing a solution obtained by a heat treatment of microbial cells under acidic conditions has been reported (International Patent Publication WO2009/88074), and the ability of this inhibitor to inhibit infection against viruses (wheat and barley dwarf viruses, soybean mosaic virus, alfalfa mosaic virus, potato leaf roll virus) is described therein. However, inhibition of infection of Tobamovirus viruses and Cucumovirus viruses with this inhibitor is not known.
Furthermore, a method of controlling diseases caused by plant pathogenic microbes (pathogenic filamentous fungi) by sprinkling an amino acid fermentation liquid, specifically, a proline fermentation liquid, on plants has been reported (Japanese Patent Laid-open No. 6-80530). However, the effect of amino acid fermentation by-produced liquids for controlling viral diseases has not been previously reported.